Project Summary This project will examine the long-term effects of lifetime cognitive engagement, the short-term effects of cognitive engagement in old age, and the mediating effect of lifetime cognitive engagement on those short- term effects. This examination will be made possible by using rich data from the large, longitudinal but complex Health and Retirement Study (HRS), including its supplements such as the Consumption and Activities Mail Survey (CAMS), the Participant Lifestyle Questionnaire (PLQ), and the Life History Mail Survey (LHMS). To make use of the data we will create measures of cognitive engagement and build a statistical model that can jointly estimate long-term associations and short-term effects. The results of our analysis will help assess the importance of the long-term effects of cognitive engagement over the life course, through the mechanism of cognitive reserve, and the shorter term effects of cognitive engagement at older ages, through ?use-it-or-lose- it?. We will develop measures of cognitive engagement, defined as the extent to which people use their brains, for a large set of subjects spanning their entire lives. To measure cognitive engagement in young and middle ages, we will use complete and harmonized histories of education and employment and detailed descriptions of jobs that include requirements of cognitive and social tasks. To measure cognitive engagement in older ages, we will use many variables on work and non-work activities, scattered around different parts of the survey and available for different subsets of the respondents. We will complement the activity-based measures by a novel measure based on response patterns to cognitively demanding survey questions, such as don't know responses, rounded and crude value responses, longitudinal variation of responses, and other aspects of response style. All of these measures make use of information on what people do in contrast with existing measures of cognitive engagement that are based on summary measures of educational attainment or self- reports about liking to engage in cognitively demanding activities. Our measures will allow us to analyze how lifetime cognitive engagement predicts interpersonal differences in cognitive engagement in old age and how health, functional limitations and depression may confound or interact with such effects. The results of our analysis will enhance our understanding of the complex role cognitive reserve may play in shaping cognitive decline and dementia in old age.